Effects of Source Redistribution on Jet Noise Shielding

Abstract

The potential of jet noise shielding from the Hybrid Wing Body (HWB) airplane is investigated in subscale experiments. The jet nozzle had a bypass ratio 10 and was operated at realistic takeoff exhaust conditions using helium-air mixtures. The shield, fabricated from a thin flat plate, had the generic shape of the HWB planform. Redistribution of the jet noise source is essential for achieving substantial noise reduction. Devices used to alter the jet noise source comprised chevrons (in mild and aggressive configurations) and a number of porous wedge fan flow deflectors. Using the estimated cumulative (downward plus sideline) EPNL reduction as a figure of merit, shielding of the plain nozzle yields a 2.4 dB reduction. Application of the aggressive chevrons increases the reduction to 6.5 dB, while the best wedge configuration improves this figure to 6.9 dB. Combination of wedge and aggressive chevrons yields a benefit of 7.6 dB. Examination of high-definition noise source maps shows a direct link between the insertion loss and the axial location of peak noise source. The aggressive chevrons cause an abrupt contraction of the noise source length at Strouhal number Sr=1.2, while the wedge induces a gradual contraction with increasing frequency. As a result, the insertion loss with the aggressive chevrons is stronger than with the wedge. However, because the wedge is inherently quieter than the chevrons, it gives a slightly better overall benefit. Surveys of the mean flow field show that the wedge, and its combination with chevrons, produces a significant reduction in the potential core length. On the other hand, the chevrons alone induce modest changes in the length of the high-speed region of the jet. Therefore, the mean velocity field by itself cannot provide useful information for inferring the noise source length for these complicated flows.

Jet-Surface Interaction Test: Far-Field Noise Results

“…Brooks and Hodgson evaluated several theories for the case of a two-dimensional airfoil [12] while Howe considered, experimentally, how the shape of the trailing edge influences the noise created [13,14]. More recently, Mayoral and Papamoschou have used experiments to study the potential for increasing jet noise shielding by redistributing the noise sources via chevron nozzles [15] and to examine jet noise shielding in a hybrid wing-body aircraft [16], Hutcheson and Brooks considered the effect of the angle of attack on trailing edge noise [17], and Lawrence et al [1] acquired near-and far-field data for a round jet near a flat surface to validate Amiet's trailing edge noise prediction method and the classically derived scaling exponents. The current Jet-Surface Interaction Tests are an effort to build on these experiments to cover a wider range of surface positions, jet flow conditions, and, in future phases, geometries.…”

Harmonic and Broadband Separation of Noise from a Small Ducted Fan

“…Notice that the errors ε k [1] and ε k [2] cannot be determined since the stencil contains points outside the domain of the complex envelope. For an order k, the matrix on the left hand side is a sparse band matrix (tridiagonal matrix in this example) with dimensions of (N − p) × N , p being the number of the filter poles (p = 2 in this example).…”

“…The rapid-prototyping approach has been used successfully with jet noise testing of realistic nozzle configurations and open-rotor spinning at full-scale tip speed powered by high-performance DC motors. 2,5 More recently, assessments of the relevance of UCI's small scale ducted fan indicate that small-scale models reproduce with good fidelity the main acoustic features 16 .…”

“…Attainment of the noise goals requires not only improvements at the component level, but also a systems integration approach for the design of the propulsor and the airframe. The shielding of engine noise by the airframe has been of particular interest, with recent efforts addressing fan inlet 1 , jet 2,3 , and open-rotor noise. 4,5 The blended wing body (BWB) airplane concept has been central to these efforts because its layout is amenable to innovative integration concepts and its aerodynamic efficiency is superior to that of conventional airplane designs.…”

Airframe Shielding of Noncompact Aviation Noise Sources: Theory and Experiment

“…In the current century, experimental and theoretical investigations were performed for clarifying features of shielding effect. [4][5][6][7][8][9] In particularly, recent experiments showed that the effectiveness of shielding effect is substantially smaller then predicted in some early works. 4,5 In theoretical works the applications of different diffraction methods were under consideration also, however, the shielding effect was mainly considered for point sources.…”